Bibtex:  added non-standard transformation rule.
Bibtex:  added non-standard transformation rule.

Transform "author" -> "bookauthor" when source = mvbook and
target = book.  Even though this isn't in the bibtex manual,
it seems to make sense in the context of converting to CSL.

jgm/pandoc-citeproc,jkr/pandoc-citeproc,adunning/pandoc-citeproc